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Reflective Solar Cooker

Grade Level: Grades 6-8 (Ages 11-13)

Subject Areas: STEM, Math, Science

This reflective solar cooker uses the Sun's energy to cook marshmallows. The target cooking area is the space where the light concentration is greatest. Never look directly at the Sun! It could damage your eyes. Don't allow the cooker to reflect sunlight into your eyes. This activity requires adult supervision.

Materials

• shoebox

• aluminum foil

• string

• tape

• straws

• manila folder

• marshmallows (white and chocolate, or other color)

Preparation

1. Cut slots of equal length down the short sides of the shoebox opposite each other. Draw a scale, beginning with zero at the top, along each slot. Then cut diagonal slits at the corners of the box for the string.

2. Cut a manila folder in half along the fold. Place one half inside the shoebox, so that the folder bows into a curved, half-pipe shape resting on the bottom of the box. Fasten with tape in this shape to the box.

3. Lay a sheet of aluminum foil, shiny-side up, along the curved folder. Tape it to the box, fitting it to the folder shape.

4. Cut two 20-inch lengths of string. Knot each at one end. Floss the knotted ends into slits A and B. Drape the string inside the box, and insert the other end into slits C and D.

Experiment

1. Place one white marshmallow onto a straw near the end, and a colored marshmallow (or more, if you have several colors) on a second straw.

2. Snip a slit at one end of the second straw and join the slit end to the other straw. Space the two marshmallows an inch or two apart from each other.

3. Lay the straw into the slot so that the marshmallows are near the center of the box. The straws should rest on the string at both ends.

4. Pull on both strings to bring the straw to the first level from the bottom.

5. Direct the box toward the Sun; prop it up. Allow the marshmallows to cook for a specified time.

6. Repeat with another set of marshmallows at a different height for the same length of time.

Analysis

1. Why is the shiny surface curved? Would this work if it were straight?

2. Did the color of the marshmallow make a difference? Why?

3. Did the height of the straws make a difference? Why?

Answers

1. It is curved to focus the sunlight. A straight surface will reflect but not focus light.

2. The darker marshmallows should cook faster, since white reflects rather than absorbs energy. (This is the reason you are cooler when you wear white clothes in the sunlight than when you wear dark colors.)

3. When the straws bring the marshmallows where the Sun's energy is most concentrated, the marshmallows will cook fastest. Imagine that the reflecting surface is part of a circular pipe (depending on the shape of your box, it may not be perfectly circular). The focus is one half of the radius of this pipe.